Standards are set in accordance with Japanese Industrial Standards (JIS) concerning processing oils, for example, coolants for use in heat treatments such as quenching processing, coolants or lubricants for use in hot processing (rolling or forging), cutting liquids and cold rolling oils, because predetermined quality and properties are required.
For example, a standard is set in JIS K2242 (standard name: heat treating oils) for the coolants used in heat treatments such as quenching processing.
In this JIS K2242, the kinds of coolants are classified in accordance with uses, and the quality and properties (cooling performance, stability) are standardized for each kind. Specifically, use is made of a surface temperature measuring device which comprises a heating electric furnace as a heater, a silver bar as a measurement target, and a thermometer. The heated silver bar is rapidly put into a sample (coolant to be inspected), and a cooling curve is obtained using a temperature change in the silver bar at this moment as a function of time. On the basis of this cooling curve, a characteristic temperature and a cooling time from 800° C. to 400° C. are measured up to 1/10 seconds, thereby obtaining cooling performance.
Thus, while the silver bar is used as the measurement target in the surface temperature measuring device conforming to JIS K2242, steel is often the metal that is actually quenched. Nevertheless, the silver bar is used as the measurement target for the following reasons.
Firstly, because steel has poor thermal conductivity and a wide temperature distribution, temperature changes in the surface of the measurement target can not be measured with good sensitivity (see C in FIG. 4). On the assumption that steel heated to a high temperature is put in the heat treating oil (coolant), a large temperature difference is produced from the surface to the center of steel in a short time. Moreover, the closer the steel surface is, the steeper temperature gradient is. The reason is that the transfer of the heat inside steel to the surface side thereof does not catch up with the release of the heat from the surface of steel due to the low thermal conductivity of steel. For example, the thermal conductivity of chrome-molybdenum steel is only about one tenth of the thermal conductivity of silver.
Secondly, the silver bar has heretofore been used because the surface of steel is oxidized under an air atmosphere.
Therefore, in the conventional metal surface temperature measuring devices, there has been a problem that the temperature change in the surface of steel during quenching processing can not be accurately measured.
The present invention has been made in view of the foregoing circumstances, and is directed to provide a metal surface temperature measuring device which makes it possible to rapidly measure the surface temperature of a metal such as steel having a low thermal conductivity in accordance with the kind of coolant and heat treatment conditions such as a heat treatment temperature.